a). Technical Field
This invention relates generally to parallel detecting spectroscopic ellipsometer/polarimeter instruments that are capable of determining the polarization state of light over a wide range of wavelengths, in particular, after the light has interacted with a sample. The parallel detecting spectroscopic ellipsometer/polarimeter provides spectroscopic polarization information by simultaneously measuring four specific polarization states of the light with an optical configuration that requires no moving parts. The spectroscopic polarization information, which is so collected, can be used to provide real-time information about the sample through the use of advanced interpretive algorithms.
b). Background Art
The polarization effects of light reflected from surfaces has been studied since the early 19th century. In general, the term ellipsometry usually applies to analysis of light reflected from a surface, where the reflected light must be in a well defined pure state of elliptical polarization. More specifically, the term xe2x80x9cellipsometer,xe2x80x9d is based on the phenomenon that the electric field vector of light reflected from a sample surface forms an ellipse in a time resolved wave due to the polarized light components parallel and perpendicular to the sample surface interacting in different ways and usually applies to analysis of reflected light where the light must be in a well defined pure state of elliptical polarization,. The term xe2x80x9cpolarimeterxe2x80x9d is less well defined, but is usually applied to analysis of transmitted or scattered light. In general polarimetry determines the complete polarization state of light, including the capability to detect non-polarized components. The change in the polarization state of light measured by an ellipsometer after interacting with a material, is extremely sensitive to the properties of the material, including the thickness of a film, its electronic energy states, its surface roughness and morphology, its composition, and defect densities.
In a typical ellipsometer installation, light in a collimated beam passes through a linear polarizer that is oriented so that the optical electric field has components parallel and perpendicular to the plane of incidence of the material sample with which it will interact. After the interaction of the light with the sample, the relative amplitudes and phases of the components parallel and perpendicular to the plane of incidence of the material sample are changed. An ellipsometer measures changes in the relative amplitudes and phases between the parallel xe2x80x9cpxe2x80x9d and perpendicular xe2x80x9csxe2x80x9d electric field components of a light beam, and more specifically of a polarized light wave, as it reflects from a sample surface. These parameters are traditionally expressed as "psgr" and xcex94, which are related to the ratio (xcfx81) of the reflectance coefficients for the p and s optical electric fields, ie. xcfx81=tan("psgr")eixcex94=rp/rs. Unlike a reflectance/transmittance measurement, which only provides the ratio of reflected/transmitted to incidence irradiances, an ellipsometer can extract both real and imaginary parts of the dielectric function of the sample, (xcex51,xcex52), as a function of photon energy, hv, from the "psgr" and xcex94. However, due to the huge computational demands and the extremely tedious measurements, polarimetry and ellipsometry did not find a large number of applications until the second half of the 20th century, when automation and computers become readily available. A good history of ellipsometry in presented by R. M. A. Azzam, xe2x80x9cSelected Papers on Ellipsometry,xe2x80x9d SPIE Milestone Series, Vol. MS 27, SPIE, Bellingham, Wash. (1991).
Since the 1960""s, literally thousands of ellipsometry/polarimetry papers and patents have been written discussing hundreds of applications and instrument designs. The advent of spectroscopic ellipsometry (D. E. Aspnes, J. B. Theeten, F. Hottier, xe2x80x9cInvestigation of the effective-medium models of microscopic surface roughness by spectroscopic ellipsometry,xe2x80x9d Phys. Rev. B, 1979) and variable incident angle measurements (O. Hunderi, xe2x80x9cOn the problems of multiple overlayers in ellipsometry and a new look at multiple angle of incidence ellipsometry,xe2x80x9d Surface Science 1976) has greatly added to the large interest as well. From an instrumentation standpoint, three basic approaches have been followed to measure the polarization state of light, xe2x80x9cnullxe2x80x9d, xe2x80x9crotating analyzerxe2x80x9d and xe2x80x9cpolarization filterxe2x80x9d ellipsometers. The first instruments that measured polarization used true nulling methods that required a phase shifter as well as a polarizer to determine the light""s polarization state. Modern day equivalent instruments determine the complex reflectance change, xcfx81, by sampling the intensity of the reflected light after it passes through a second polarizer, called the xe2x80x9canalyzerxe2x80x9d, whose orientation changes in a continuous fashion. The detected signal is a sinusoid as a function of the orientation angle (R. W. Collins, I. An, H. V. Nguyen, and Y. Lu, xe2x80x9cReal-time spectroscopic ellipsometry for characterization of nucleation, growth, and optical functions of thin filmsxe2x80x9d, Thin Solid Films, V. 233, (1993), p. 244). Although phase and amplitude of that sinusoid can be analyzed to extract "psgr" and xcex94, the data needs to be acquired as a function of time, with each analyzer orientation taking a finite time interval to acquire. Even with a continuously rotating analyzer and triggered detection system, data are acquired serially or sequentially as a function of the phase angle of the analyzer. This type of system can acquire spectroscopic data, primarily by passing incident white light through a monochrometer that provides a single wavelength of light. This single wavelength of incident light directed through a polarizer is reflected off of the sample, then through an analyzer and is detected with a photodetector. Additional wavelengths are selected with a monochrometer, and the ellipsometry data is serially acquired as a function of those wavelengths. For either single wavelength or multiple wavelength (spectroscopic) cases, the main problem with the use of such xe2x80x9crotating analyzerxe2x80x9d instrinents is that the data is acquired serially, and typically takes several seconds to measure a data point at a single wavelength.
Polarization filter instruments primarily related to polarimetry, divide the light into multiple components to enable a complete or partial determination of the polarization state. In the 1970""s photopolarimeters were designed for astronomy applications in which modulation of polarizing optics were used to determine polarization states of the light. Several photopolarimeter instruments based on simultaneous measurement of different polarizations of the light have been designed since then, including R. M. A. Azzam. xe2x80x9cDivision of amplitude photopolarimeter for the simultaneous including R. M. A. Azzam, xe2x80x9cDivision of amplitude photopolarimeter for the simultaneous measurement of all four Stokes parameters of light,xe2x80x9d Optica Acta Vol. 29, pg. 685 (1982), G. E. Jellison, xe2x80x9cFour channel polarimeter for time resolved ellipsometry, Opt. Lett., Vol. 12, Pg. 766 (1985), Azzam U.S. Pat. No. 4,681,450; Siddiqui U.S. Pat. No. 5,081,348; Berger et al. U.S. Pat. No. 5,102,222; Yamada et al. U.S. Pat. No. 5,335,066, and Lacey et al. U.S. Pat. No. 5,793,480. The basic polarization filter designs involve polarization dependent splitting of the light into several components that are measured simultaneously with multiple detectors. The collected intensities are then analyzed to determine the polarization state of the incoming light.
Azzamn and Jellison, supra, have developed photopolarimeters that split the light to be analyzed into different beams using appropriately coated optics. The output signals are collected with detectors, primarily photodetectors that do not discriminate frequency, and some or all of the parameters that characterize the polarization state of the monochromatic light is calculated using the measured intensities and an instrument matrix. Typically, the detected polarization intensities are linearly related to the four xe2x80x9cStokesxe2x80x9d parameters that more traditionally characterize the polarization state of the light. Through a calibration operation the 16 coefficients of this linear relationship can be determined. The coefficients can be represented by a 4xc3x974 matrix, referred to as the xe2x80x9cinstrument matrixxe2x80x9d that compactly expresses the transformation of the detected intensities into the four Stokes parameters.
Azzam U.S. Pat. No. 4,681,450 discloses an optical polarimeter that uses the reflection off three different photodetectors to change the polarization state of the single wavelength light for the next photodetector. Siddiqui U.S. Pat. No. 5,081,348 discloses an optical polarimeter having four channels which provides a method and apparatus for rapidly determining the polarization state of an incoming beam of single wavelength light, primarily from an optical fiber; in which four separate portions of the beam are passed simultaneously through four Stokes filters. Berger et al. U.S. Pat. No. 5,102,222 discloses a system for single wavelength light wave polarization determination using a hybrid system for determining polarization vector components (Stokes parameters) that includes beam splitters. The use of two splitters and four detectors allows reconstruction of the polarization state.
Of greatest interest is Yamada et al. U.S. Pat. No. 5,335,066, which discloses an ellipsometric system and method. In the claimed invention, the light source is taught to be a single wavelength linearly polarized laser beam, that is reflected from a sample as a beam which is passed through splitters and polarizers so as to detect the polarization state of the light by means of four intensities. These intensities are converted by the associated software into "psgr" and xcex94 at the single wavelength of the laser. In use, the smallest detected intensity is discarded when determining ellipsometric parameters, e.g. surface conditions. However this limits the amount of information obtained since the complete polarization state of the light is not determined.
Lacey et al. U.S. Pat. No. 5,793,480 discloses a combined interferometer/ellipsometer for determining the space between a transparent member, such as a glass disc, and a reflective surface, and for determining the real and imaginary parts, n and k, of the index of refraction. The single wavelength reflected light is divided into separate beams; two beams are used for interferometry measurements to determine the air gap, while a third is used in conjunction with the other two to determine n and k.
While all of the division of amplitude systems in the prior art discussed to this point can collect data over a range of wavelengths, they can only collect one wavelength at a time and require separate calibration at each wavelength. Only two of the references teach about collecting multiple wavelengths simultaneously with division of amplitude instruments. These are Azzam U.S. Pat. No. 5,337,146 and T. Todorov and L. Nikolova, xe2x80x9cSpectrophotopolarimeter: fast simultaneous real-time measurement of light parameters,xe2x80x9d Optics Lett., Vol. 17, pg. 358 (1992), which teaches how a grating can be used in conjunction with other polarizing optics to determine the polarization state of light by using a technique that involves measuring multiple order diffraction reflections with four or more detectors. The diffraction grating separates the light into individual frequency components, so that if additional photodetectors or photodetecting arrays are used, then multiple wavelength data may be obtained. The main problem with using these systems in their preferred embodiment is that they have substantial position and alignment sensitivity that will be associated with the spatially resolved light after it is reflected from the diffraction grating. While this problem may be overcome at a single wavelength, it is virtually impossible to solve for simultaneous detection of multiple wavelengths.
Several parameters, such as the polarizer azimuth and the angle of incidence, have to be determined with a calibration procedure before the ellipsometer can be used effectively. Several calibration methods, e.g. Tompkins, Harland and McGahan, William, Spectroscopic Ellipsometry and Reflectometry, John Wiley and Sons, Inc., 1999, have been used in the past for different ellipsometers, but the different optical components used for each type requires an unique calibration procedure. Typically, calibration procedures for polarimeters and spectroscopic ellipsometers are tedious and time consuming. For example, the calibration methods discussed by Azzam, R. M. A and Masetti, E. and Elminyawi, I. M. and EI-Saba, A. M., xe2x80x9cConstruction, calibration, and testing for a four-detector photopolarimeterxe2x80x9d, Rev. Sci. Insrun., Vol. 59, 1988, pp. 84-88 and Azzam, R. M. A. and Lopez, A. G., xe2x80x9cAccurate calibration of the four-detector photopolarimeter with imperfect polarizing elementsxe2x80x9d, J. Opt. Soc. Am., Vol. A 6, 1989, pp 1513-1521, namely the xe2x80x9cfour-point calibration methodxe2x80x9d and the xe2x80x9cequator-poles calibration methodsxe2x80x9d, are time consuming since they are only appropriate for calibrating a single wavelength at a time. In the four-point calibration method four linearly independent input states are used for calibration at a given wavelength. Thus the procedure has to be repeated for each of the wavelengths under consideration. In the equator-poles method the output intensities are recorded as a function of linearly polarized light azimuth over one period for a given wavelength and fitted to a 3-term Fourier series. Again, this procedure has to be repeated for all the wavelengths of interest and cannot be used for broad wavelength applications.
It is thus seen that the calibration procedures of the ellipsometers of the prior art are complex and inefficient, and that they are incapable of simultaneously determining the calibration parameters for the entire spectral range of the instrument, and they require a substantial amount of time for each calibration. More specifically, none of these prior art references teaches or suggests a combined parallel detecting, spectroscopic ellipsometer/polarimeter sensor system using four state polarization filtering in a manner which substantially decreases spectroscopic polarization data collection time, which also provides true multispectral analysis of polarization data, in real-time, using advanced transformation algorithms for thousands of wavelengths simultaneously, nor are of a design which allows them to be integrated within a processing chamber. In addition, none of the state-of-the-art spectroscopic ellipsometers are capable of polarimetry or simultaneous collection of polarization state information. Accordingly, there exists a need for a spectroscopic ellipsometer/polarimeter system that provides these capabilities.
It is thus an object of the present invention to provide a system and calibration procedure for a parallel detecting spectroscopic ellipsometer/polarimeter which is simple and efficient, and capable of simultaneously determining the calibration parameters over a wide spectral range.
It is a further object of the present invention to provide such a system and calibration procedure for a parallel detecting spectroscopic ellipsometer/polarimeter that requires a fraction of the time compared to the calibration of other polarimeters and spectroscopic ellipsometers.
It is yet a further object of the present invention to provide such a system and calibration procedure for a parallel detecting spectroscopic ellipsometer/polarimeter that is performed in three steps, as detailed below, and which minimize the need for recalibration.
The present invention consists of a parallel detecting spectroscopic ellipsometer/polarimeter instrument that completely determines the polarization state of light. In preferred embodiments the light that is analyzed has a wavelengths from about 200 nm to about 5000 nm after the light has interacted with a sample, although light of other wavelengths can be analyzed. The ultimate spectral range is dependent upon available and future spectrometers and spectroscopic measuring techniques. Thus the spectral range is limited only by available detectors and is not intrinsically limited by the instrument itself. Thus, one could apply the same parallel and spectroscopic detection system for higher and lower light energies, including ultraviolet, x-rays, synchrotron radiation, far IR and even longer wavelength applications.
The parallel detecting spectroscopic ellipsometer/polarimeter system of the present invention provides spectroscopic polarization information by simultaneously measuring four specific polarization states of the light with an optical configuration that has no moving parts. The spectroscopic polarization information is collected in as little as 5 ms (limited only by detector technology, not the design of the instrument) and provides real-time information about the sample through the use of advanced interpretive algorithms in less than 20 ms. The parallel detecting, spectroscopic ellipsometer/polarimeter system of the present invention includes, generally, an electromagnetic radiation beam, including a wide spectrum of wavelengths from a single collimated source, the light beam. The light beam from the single collimated light source is directed through polarizing optics and thence impinges upon a surface that is being studied or monitored, typically a thin-film on a substrate support. The light beam that is reflected from the surface is then split into two beams by a first beam splitter. Then, a second and third splitter further splits each of those two beams into a total of four beams. The four resulting beams are then directed to pass through different polarization filters, and/or quarter wave plates to provide data for monitoring the condition of the surface of the sample from which they were reflected, all in real time.
As set forth in greater detail below, the system and its components are designed and sized to fit within a thin film processing chamber, such as a vacuum deposition chamber. It could be made even smaller, for example using integrated optics. The system is capable of supplying, in real time, any of the various valuable units of information that can be extracted through the technique of spectroscopic ellipsometry, such as monitoring the rate of growth, and/or the thickness, the microstructure, the surface morphology, the presence of voids, the existence of fractures, the composition of the sample, combinations of the foregoing, and so on, of the sample, for example during its formation by any film formation process. Ancillary to the process, human or software monitoring of thin-film growth, and termination of the thin film formation process may be practiced when the desired film properties or thickness are detected.
The parallel detecting spectroscopic ellipsometer/polarimeter of the present invention provides four combined features into a single system. These include the analysis of four channel polarization data using different polarization filters, and/or quarter wave plates, and of a system for multispectral processing of the four channels. Then, advanced transformation algorithms are used to subject the raw spectral data to provide specific and detailed analysis, in real-time. In the practice of the present invention, all of these components are assembled into a compact unit that is designed to allow it to be mounted inside of, for example, a thin film processing chamber. By combining all four of these features into a single compact package, the parallel detecting spectroscopic ellipsometer/polarimeter of the present invention provides advanced and unique capabilities that no currently known instrument is capable of performing. These capabilities include better than an order of magnitude decrease in spectroscopic polarization data collection times, as compared to state-of-the-art spectroscopic ellipsometers and true spectroscopic analysis of polarization data at thousands of wavelengths simultaneously, as compared to single wavelength division of amplitude polarimeters. The parallel detecting spectroscopic ellipsometer/polarimeter of the present invention is also of such a design and size that it is capable of being integrated within a standard size thin film processing chamber. This enables it to be inexpensively adapted for a variety of materials applications, even if the processing chamber in which it is placed had been originally designed without external optical access to the sample. The sensor system of the present invention is referred to as a parallel detecting, spectroscopic ellipsometer/polarimeter, because it measures the full Stokes vector, that is all four Stokes parameters, that can be translated into "psgr", xcex94, intensity, and depolarization information for thousands of wavelengths. Thus, the instrument is also capable of polarimetry, of which ellipsometry is a subset or species.
The parallel detecting spectroscopic ellipsometer/polarimeter of the present invention detects four polarization states that can be expressed in terms of the four Stokes parameters:
S0=as2+ap2, S1=as2xe2x88x92ap2, S2=asap cos(xcfx86pxe2x88x92xcfx86s), S3=asap sin(xcfx86pxe2x88x92xcfx86s)
where, a and xcfx86 are the amplitude and phase, and s and p denote optical field orientation in the planes parallel and perpendicular to the plane of incidence, respectively. To determine the four Stokes parameters, intensities from the four spectrometers are linearly transformed using a pre-calibrated, 4xc3x974, xe2x80x9cinstrumentxe2x80x9d transfer matrix ({right arrow over (I)}=A{right arrow over (S)}). A data set, consisting of the four Stokes parameters as a function of wavelength, can then be used to determine complex reflectance and/or "psgr" and xcex94, and also provides information about the depolarization of the light, a capability no other rotating polarizer based spectroscopic ellipsometer possesses. Depolarization information can be used to more easily and directly measure surface roughness, interfacial mixing, and other features that cause the light to become unpolarized. This choice of polarization state measurements is significantly different from present state-of-the-art spectroscopic ellipsometers that measure only polarized light in a serial fashion and cannot measure depolarization directly. Thus, the present invention is the first known instrument to measure the complete polarization state of the light, including depolarization, spectroscopically. The parallel detecting of the polarization states significantly decreases the time required to make a measurement and eliminates the random and systematic errors associated with signal drift or interference that occur with conventional ellipsometry measurements taken over a period of time. Therefore, the simultaneous detection of the present invention significantly enhances the intrinsic precision of spectroscopic ellipsometry. Furthermore, by simply modulating the incident polarization state, the system of the present invention is capable of measuring the entire Mueller matrix of the sample at each wavelength, rather than only measuring the Stokes vector of polarized light reflected from a sample. This is of substantial value as the Mueller matrix provides additional data that enables the determination of the polarization modification characteristics of the sample, and this data can then be translated into valuable information for materials engineers. In addition, the system of the present invention also measures the intensity of the light after it interacts with the sample. Therefore, by simply measuring the incident intensity of the reflected light, the sensor system of the present invention also provides spectroscopic reflectometry measurements.
As noted above, the sensor system of the present invention provides multispectal wavelength analysis of four channel polarization data, as compared to existing polarimeters that only measure polarization of a single wavelength at a time, defined, for example by a filter at the light source or by a laser source. The multispectral wavelength nature of the parallel detecting spectroscopic ellipsometer/polarimeter sensor system of the present invention required unique design innovations. These include accurate alignment of collected light into four polarization-state channels that deliver the multi-wavelength light into spectrometers, the removal of wavelength dependence of the optical components from the measurement, and the removal of, or the need to calibrate, polarization changes as a function of wavelength from optical components and geometries when making the measurement. This is accomplished in preferred embodiments of the parallel detecting spectroscopic ellipsometer/polarimeter of the present invention in two ways. First, a calibration scheme is provided that removes the polarization and wavelength dependence of the optical components and provides an intensity independent normalization that does not require prior knowledge of the angle of incidence. Then, a mechanism, such as fiber optic cables coupled through position desensitizing optics, is used to collect the individually polarized multi-wavelength intensities and deliver them to the spectrometers. While it is possible to place the spectrometers directly with collection optics, the use of fiber optics enables the spectrometers, that may be more sensitive to the harsh environments where the polarization measurements are taking place, to be decoupled from the parallel detecting spectroscopic ellipsometer/polarimeter detection head, and therefore removes the requirement for accurate alignment of the spectrometers to the detection optics. This means that, for the first time, polarization measurements with a parallel detecting spectroscopic ellipsometer/polarimeter can be made close to samples in harsh environments, e.g. inside a vacuum chamber which has significantly elevated temperatures, for example due to high energy deposition of materials and resulting substrate heating. No currently known polarimeter or ellipsometer provides this flexibility while also providing the complete determination of the optical properties of the sample which is undergoing analysis, as does the parallel detecting spectroscopic ellipsometer/polarimeter of the present invention. All other instruments require an accurately aligned optical window to perform measurements inside a processing chamber. This adaptability and complete characterization capability of the parallel detecting spectroscopic ellipsometer/polarimeter sensor of the present invention enable it to be used for new applications previously not allowed by existing polarimeters and ellipsometers due to their strict geometric and physical limitations. In addition, the parallel detecting spectroscopic ellipsometer/polarimeter sensor of the present invention can be installed into processing chambers that were originally not designed to accommodate optical monitoring.
In addition to providing complete measurement of optical properties in a parallel detecting spectroscopic ellipsometer/polarimeter sensor having no moving parts and in real time, the sensor of the present invention is designed to include a multitude of unique alignment concepts. These include the use of a dual laser/quadrant photodiode system that establishes a well-defined geometrical relationship between the sample and the optical heads. This ensures accurate handling of the optical beam within the head. In addition, CCD (charge couple device) cameras may be used for alignment of the actual reflected beam. Furthermore, the use of a corner cube and double reflection from the sample eliminates the need for a source head by combining both the source and detection head into a single unit. This further simplifies the optical access to the sample and makes it possible to adapt the parallel detecting spectroscopic ellipsometer/polarimeter to chambers that may preclude installation within the processing environment. Finally, the use of diffusers, defocusing, and physically larger source beams decrease alignment sensitivity while maintaining measurement precision and accuracy.
As noted above, the use of advanced transformation algorithms is also an essential component of the parallel detecting spectroscopic ellipsometer/polarimeter system of the present invention. Such advanced transformation algorithms provide real-time intelligent process control. These algorithms are based on a number of increasingly complex interpretation schemes, including models. Such exact models are developed from first principles with data reduction. An empirical model may be created from exact optical property measurements from actual materials of interest. Models may be developed from combinations of both exact and empirical models. Models may be developed using data transformation: which is based on regression algorithms, or on spectral fingerprint type analysis, or on heuristic algorithm training techniques, e.g., neural networks. The specific interpretation scheme used with the models will depend upon the specific material properties needed, and where appropriate, for intelligent process control and required on-line quality control.
Not only can standard polarimetry, ellipsometry, and reflectometry information be provided by the parallel detecting spectroscopic ellipsometer/polarimeter of the present invention at multiple incident angles, but additional configurations can be used to obtain additional information. Some of these configurations include replacing the spectral distribution with information delineated according to other characteristics. For example, reference to multiple angle collection of the outgoing light from the sample can be used to perform forward scattering analysis of the surface interaction, thereby providing valuable information about the surface topography, grain structure, and other contributions to scattering.
The system may also image an illuminated line on the sample surface onto up to four CCD cameras, thereby detecting the light through up to four polarization-filtered channels. This would yield a spectral fingerprint of the surface along the line and having spatial resolution that could approach 5 microns. By extending this idea to a general light collector, while giving up the spectral data, one could obtain a two-dimensional image of the polarization modification characteristics of the sample and therefore perform a broad range of imaging polarimetry measurements. Additional combinations of optics and optical properties integrated directly into the fiber optics could result in additional size reduction and optical path simplification.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description, showing the contemplated novel construction, combination, and elements as herein described, and more particularly defined by the appended claims, it being understood that changes in the precise embodiments to the herein disclosed invention are meant to be included as coming within the scope of the claims, except insofar as they may be precluded by the prior art.